RESUMO
OBJECTIVE: To examine the geometry of the proximal isovelocity surface area (PISA) envelope and its associated isotach, and to evaluate the accuracy of two models of calculating volumetric flow by using the PISA technique. DESIGN: A new model for determining isotach geometry from the PISA envelope was developed and tested in an in vitro simulation. SETTING: Echocardiography Laboratory, Hotel Dieu Hospital, Kingston, Ontario. MATERIALS AND METHODS: PISA envelopes were visualized using an in vitro flow simulator with a series of sharp-edged orifices (2.5 to 16 mm diameter) at a range of flow rates (10 to 110 mL/s). INTERVENTIONS: Flow calculations based on the traditional hemispherical geometric assumption for the isotach and the new model were made and compared with measured flow rates. MAIN RESULTS: The hemispherical model systematically and significantly underestimated flow. The nonhemispherical model, which requires measurement of both the height (a) and lateral width (2d) of the PISA envelope, provided improved estimates of flow. CONCLUSIONS: The nonhemispherical model provides a better estimate of flow through an orifice. Flow rate Q can be calculated directly from the size of the PISA envelope and the aliasing velocity (VA) by using the relationship Q = (3.14d2 + 5.97da + 1.37a2)VA or can be read from a nomogram.
